Optical information read/write device

Abstract: Both guide grooves and lands of an optical information recording medium constitute information tracks, the depth of the guide groove being between approximately λ/8 and approximately λ/4.7 in optical length and the width of the guide groove being between 0.34R and R, where R represents the beam diameter of a write laser beam on the record film and the power for writing information on each guide groove being differentiated from that for writing information on each land.

Description: The invention relates to an optical information write/read system in which information data is written on a recording medium by a write laser beam, and written information data is read from the recording medium by a read laser beam. The invention particularly relates to an optical information read/write device in use for recording information signals on an optical information recording medium having improved information density, where information is written on guide grooves as well as regions between two guide grooves adjacent with each other, all guide grooves of the optical recording medium being formed before writing information. In recent years there have been extensive research and development on optical information recording media to achieve high density recording of such information as image and voice signals. One of the well-known examples is an optical disk. A writable optical disk has guide grooves formed in advance by cutting on a transparent substrate. Information signals are read or written by focusing a laser beam onto these tracks. When a commercially available optical disk is used for writing information signals, signals are normally written on either guide grooves or regions between two guide grooves adjacent with each other (hereinafter referred to as the land), but not on both. When writing information signals, a laser beam from an optical head is focused by an object lens onto a recording layer in a guide groove. When the intensity of the laser beam is strong, the region the laser beam is focused on undergoes a change in reflectance, and a record mark is formed. The distance between record marks on adjacent guide grooves is the width of the land between them. When reading information signals, record marks arranged along a guide groove are irradiated by a focused laser beam, which is either reflected or absorbed, thus the intensity of the reflected laser beam varies. Reading of information signals is performed by an optical head detecting the variation of the intensity of the reflected light. The track pitch, that is, the spatial period of guide grooves is about the size of the laser beam spot and set to be 1.6 µm in this explanation. The depth of the guide groove is about one eighth of the wavelength of the read beam in optical length. This depth gives rise to the maximum push-pull signals of reflected light from an optical disk having guide grooves, thus provides the most stable push-pull tracking control. In order to increase memory capacity of an optical disk it would be natural to write information signals on both guide grooves and lands and increase information track density. In this method, however, the distance between two adjacent information tracks is approximately a half the laser beam spot. Therefore, the beam overlaps the information tracks next to the information track intended to read. Thus, the cross talk for reading becomes significant causing degradation of the read S/N ratio. A paper of SPIE Vol. 1316 Optical Data Storage (1990), pp. 35, "High track density magneto-optical recording using a cross talk canceller" describes a method to reduce the cross talk, in which an optical read apparatus comprises triple-beam optics and a cross talk cancellation circuit. A drawback, however, is that the optics and signal processing circuits become large and complex. A US Patent 4,423,502 (or GB-A-2091028 which belongs to the same patent family) presents a technique to reduce cross talk between adjacent information tracks by using different effective levels for adjacent information tracks. When information is written on both guide grooves and lands, the difference between the effective levels means the depth of the guide grooves. According to the US Patent, when the writing mechanism is to change the reflectivity of the medium, the desirable effective level difference is between λ e /8 and λ e /4 (λ e is the wavelength of the read laser beam at the recording medium). This method allows one to reduce the cross talk without using the cancellation circuits mentioned above. JP-A-5282705 discloses an optical information recording medium having lands and grooves on which information are recorded. This document teaches that cross talk can be reduced by setting the groove depth within the range λ / 7 - 5λ / 14 . This prior art is reflected by the preamble of claim 1. This patent, however, mentions nothing about erasable media. We applied the method to an erasable optical disk and found the following problems. (1) When the width of the guide groove is narrower compared to the read beam spot, the beam erases part of marks on the adjacent tracks causing degradation of the read signals. (2) When the width of the guide groove is wider compared to the write beam spot, the write beam tends to wobble from the center line of the guide groove. It causes incomplete erase of old marks when writing in the over-write scheme, thus resulting again in degradation of the read signals. (3) If the thermal conductivities of medium materials are high, or the structure of the medium makes the thermal conductivity high, the heat generated in writing erases part of marks on the adjacent tracks giving rise to degradation of the read signals. The object of the present invention is to present optical recording media and optical read/write devices which allow higher effective information density while suppressing degradation of read signals. To achieve this object the present invention provides an optical information write/read system as specified in claim 1. Preferable embodiments of the invention are defined in the dependent claims. With this configuration, in the system, the write beam irradiates the full widths of the guide groove and land. Consequently, the widths of the formed record marks are always narrower than W g , and do not extend to the adjacent information tracks. Therefore, when writing, the beam erases old marks completely, but does not erase even part of marks on the adjacent information tracks, thus giving rise to no degradation of write signals. Further a more desirable medium uses as a record film a material which undergoes a phase transition between an amorphous phase and a crystalline phase, and has an arrangement of a dielectric film, record film, another dielectric film, and reflection film, in this order, the record film being between 5 nm and 70 nm in thickness and the reflection film exceeding 10 nm. This arrangement suppresses the thermal diffusion within the record film and the thermal effect on the adjacent information tracks. The optical information read/write device of the present invention comprises one of the aforementioned optical information recording media, an optical system irradiating an optical information recording medium with an optical beam emitted from a light source, optical beam control means for switching the intensity of the optical beam between the write power level and the erase power level according to write signal waveforms when writing information signals, first means for moving the optical beam along the direction normal to the track, second means for relatively moving the optical beam along the direction of the tracks, means for detecting the lateral deviations of the optical beam from the information track by measuring the reflected beam from or transmitted beam through the optical information recording medium and putting out signals associated with the deviation referred to as first signals, means for receiving the first signals, inverting the polarity of the first signals so that the polarity, when the optical beam is on guide grooves, is opposite to the polarity when the optical beam is on regions between two guide grooves adjacent with each other, and putting out the modified signals referred to as second error signals, and tracking control means for receiving the second error signals and controlling the optical beam moving means so that the optical beam correctly traces the information tracks. When writing information signals, the optical beam control means switches the intensity of the optical beam between the write power level and the erase power level according to write signal waveforms. Therefore, this configuration allows one to write new record marks erasing old record marks simultaneously. Thus the optical information read/write device is able to write information signals on the optical information recording medium of the present invention in high quality and in high speed. These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: FIG. 1 shows a top view of a part of the optical disk used in the preferred embodiments of the present invention; FIG. 2 illustrates a magnified perspective view of said optical disk; FIG. 3 shows a magnified cross section of said optical disk; FIG. 4 shows the relationship between the write beam spot size and the cross sectional dimensions of the guide grooves and lands of said optical disk; FIG. 5 illustrates a schematic view to explain the suppression of the cross talk of said optical disk; FIG. 6 shows the relationship between the total reflected light intensity and the depth of the track (measured in the optical length) of said optical disk when the adjacent tracks have no record marks; FIG. 7 shows the relationship between the total reflected light intensity and the depth of the track (measured in the optical length) of said optical disk when the adjacent tracks have record marks; FIG. 8 shows the relationship between the difference of the total reflected light intensities and the depth of the track (measured in the optical length) of said optical disk; FIG. 9 shows the relationship between the cross talk and the depth of the track (measured in the optical length) obtained by calculation based on the wave optics; FIG. 10 illustrates how old record marks are incompletely erased by a wobbling beam; FIG. 11 shows a magnified top view of the optical disk illustrating erasure of marks on the adjacent track; FIG. 12 shows the error rate characteristic as a function of the ratio of the guide groove width to the write beam spot diameter in the first preferred embodiment; FIG. 13 illustrates a magnified cross sectional view of the optical disk in the second preferred embodiment; FIG. 14 shows a measured result of the error rate of the read signals as a function of the thickness of the record film in the second preferred embodiment.; FIG. 15 shows a measured result of the error rate of the read signals as a function of the thickness of the record film in the third preferred embodiment; and FIG. 16 shows a measured result of the error rate of the read signals as a function of the thickness of the reflection film in the fourth preferred embodiment. The invention will be described in detail with reference to an erasable optical disk as an optical information medium. FIG. 1 shows a top view of a part of the optical disk used in the preferred embodiments. Guide grooves 101, 103, and 105 have been formed in advance in spiral shape on the optical disk and regions 102 and 104 between the guide grooves are the lands. Their widths are drawn exaggeratedly. FIG. 2 illustrates a magnified perspective view of the optical disk, in which both guide grooves and lands are used for information tracks. In FIG. 2 a layer 110 is a record layer which includes a material such as phase transition materials having different reflectivity in the amorphous and crystalline phases. This property allows one to write information signals by varying the intensity of a laser beam. Reference numerals 112, 114, and 116 are guide grooves formed on the optical disk and 113 and 115 are lands, the widths of both being almost equal to each other. A reference numeral 117 is a transparent substrate made of polycarbonate, for example. The laser beam is incident on the substrate from the bottom. A reference numeral 118 is a record mark created in the guide groove or land. The record mark is defined as a region of record layer 110 which is turned to the amorphous phase by the laser beam irradiation. FIG. 3 shows a magnified cross section of the optical disk along the points A and B in FIG. 1, mainly illustrating a structure of the record layer. A reference numeral 121 is a transparent substrate. Glasses and resins are used for transparent substrate 121. More particularly, transparent glasses, quartz, polycarbonate, polymethylmethacrylate, etc. are ordinarily used. A reference numeral 122 represents a first protection film, 123 a record film, and 124 a second protection film. First and second protection films 122 and 124 are made of dielectric materials such as oxides of metalloids, nitrides, chalcogenides, carbides, and their mixtures, more specifically, SiO 2 , SiO, Al 2 O 3 , GeO 2 , In 2 O 3 , TeO 2 , TiO 2 , MoO 3 , WO 3 , ZrO 2 , Si 3 N 4 , AlN, BN, TiN, ZnS, CdS, CdSe, ZnSe, ZnTe, AgF, PbF 2 , MnF 2 , NiF 2 , SiC or their mixtures. Particularly, ZnS, SiO 2 , Si 3 N 4 , AlN, TiN, a mixture of ZnS and SiO 2, and a mixture of ZnSe and SiO 2 are suitable because they easily grow a film and have a good thermal stability. Record film 123 is made of alloys of such elements as Te, Se, Sb, and Ge, which undergo a phase transition between an amorphous phase and a crystalline phase, or between two different crystalline phases. More specifically, GeSbTe, InSnTe, GaSb, InGaSb, and AgSbTe are suitable because writing information signals is relatively easy with these materials. A reference numeral 125 is a reflection film, made of a metal such as Au, Al, Ti, Ni, Cr, etc. or their alloys. Some optical disks may not include a reflection film. A reference numeral 126 is an overcoat made of a UV hardening resin. FIG. 3 is drawn with the vertical direction (the direction of the film thickness) more magnified than the horizontal direction (the direction of the disk radius). FIG. 4 shows the relationship between the write beam spot size and the cross sectional dimensions of the guide grooves and lands of the optical disk in the preferred embodiments. The widths of the leveled portions of the guide grooves and lands are denoted as W g . The beam spot diameter is defined as the diameter at which the intensity of the beam reduces to 1/e 2 of the intensity of the center of the beam and is denoted as R. In the following preferred embodiments we assume W g to be 0.7 µm. The boundary between the guide groove and land forms a slope, which is referred to as the taper portion. The width of the taper portion is approximately 0.1 µm. The depth of the guide groove is approximately 84 nm, which corresponds to the optical length of 0.13 µm with the index of refraction of polycarbonate being 1.58. Next the method of writing information signals on the phase transition optical disk will be described below. In the preferred embodiments of the present invention record marks are created on the record film by modulating the laser beam power between the erase and write levels depending on input signals. Referred to as the single-beam overwrite, this scheme writes new record marks on top of old ones. The advantage of this scheme is high speed of writing due to simultaneous write and erase. A detailed description of the single-beam overwrite is found in the paper, "High Speed Overwritable Phase Change Optical Disk Material", Japanese J. Appl. Phys., Vol. 26 (1987) Supplement 26-4. The reason for so setting the depth of the guide groove will be described in detail. FIG. 5 is a schematic view of the optical disk. Now assuming that a beam is tracing guide groove 132, we analyze the cross talk from lands 131 and 133. The depth of the guide groove gives rise to an optical phase difference between land 131 and guide groove 132. We also assume that there is no record mark at positions on lands 131 and 133 right next to the point A, while there are record marks 134 and 135 next to the point B. In order to make the description simpler we further assume that the reflectance of the record mark is 0% and that of the part having no record mark is 30% and that there is no record mark on guide groove 132. When the beam spot is at the point A, the beam is reflected from lands 131 and 133 and guide groove 132. If the depth of the guide groove were zero, the simple sum of the intensities of the individual reflected beams would be the total intensity of the reflected beam to be observed, which is denoted as I 0 . As the depth increases, interference between the reflected beams sets in due to their phase difference and the intensity observed by a photo detector reduces. When the phase difference is close to π, the cancellation becomes large and the total intensity of the reflected beams becomes less than the intensity of the beam reflected from only the guide groove. When the phase difference is π, that is, when the depth of the track is λ/4 in optical length, the total intensity is minimal. FIG. 6 shows the relationship between the depth of the track and the total intensity of the reflected beams. When the beam spot is at the point B, there is no reflection from lands 131 and 133 because the record marks have zero reflectance. The total reflected intensity comes from only guide groove 132. Thus the total intensity is independent of the track depth. FIG. 7 shows the relationship between the depth of the track and the total intensity of the reflected beams. The cross talk is regarded as the difference in the total intensity of the reflected beams between a case in which a point has record marks directly next to it on the adjacent information tracks and another case in which the point does not have record marks on the adjacent information tracks. Therefore, the difference between FIG. 6 and FIG. 7 represents the cross talk. FIG. 8 shows the relationship between the difference of the two figures (absolute value) and the track depth. It indicates that as the depth increases from zero, the cross talk decreases, reaching zero when the total intensities from the points A and B coincide with each other. Therefore, if the guide grooves are formed with the depth so determined, the cross talk will be minimal. FIG. 9 shows the relationship between the guide groove depth in optical length and the cross talk obtained by simulation based on scalar wave optics using the configuration and parameters of the actual optical head and the shape of the guide groove. We followed the method described by V.J. Jipson and C.C. Williams in "Two-dimensional modeling of an optical disk read out", Applied Optics Vol. 22, No. 14, pp. 2202 (15 July, 1983). The wavelength λ was assumed to be 780 nm, the pitch of the guide grooves 1.6 µm, the widths of the guide groove and land 0.7 µm, the NA of the focusing lens 0.55, and the period of record marks 6.6 µm with its shape being an oblong of the width of 0.7 µm. The reflectance of the record mark was assumed to be 15% and that of the region with no record mark 30%. The simulation indicates that the minimal cross talk occurs at the guide groove depth of 130 nm or λ/6. When the depth is in the range between 100 nm and 160 nm (approximately λ/8 - λ/4.7), the cross talk is less than -26 dB which is the normally allowable upper limit. Particularly, when it is between 110 nm and 150 nm (approximately λ/7 - λ/5.2), the cross talk is less than -30 dB, which provides a good margin. The actual reflectance can be different from those mentioned above. After intensive study to solve the drawbacks of the prior art, we found that the scheme of writing information signals on both guide grooves and lands is successfully applicable to erasable optical information storage media by specifying the shape of the guide grooves on the media and the structure of the media. First, W g is set so that Inequality (1) is satisfied. 0.34 ≤ W g /R ≤ 1.0 Let the wavelength of the laser beam be λ and the numerical aperture of the object lens in use for focusing the laser beam NA. Then the diameter R (the diameter at which the intensity of the laser beam reduces to 1/e 2 of the intensity of the center of the beam) of the write beam spot on the record surface is given by Equation (2), if the laser beam has a uniform intensity distribution. R = 0.82 x λ/NA When W g /R is greater than 1, that is, when the width of the guide grooves or lands is wider than the write beam spot diameter R, the differential tracking error signals are reduced, and tracking deviation increases even if tracking control is engaged. In the single-beam overwrite scheme incomplete erase may occur resulting in increase of jitters of read signals and hence error rate of read signals. As the record mark written by the beam spot having the beam diameter R has a width of approximately R also, incomplete erase of old record marks will occur as shown in FIG. 10. The wider W g is in comparison with R, the more the deviation of the beam spot is, causing the error rate to increase proportionally to W g . If W g /R is less than 0.34, 1/3 of the beam spot overlaps on the adjacent track. In the phase transition type optical disk formation of a record mark starts at the center of the beam spot spreading toward the circumference and stops normally at the taper portion of the guide grooves. If 1/3 of the beam spot overlaps with the next track, however, a record mark is created on the next track causing partial erase of a signal there and giving rise to increase of the read error rate. In the present invention an appropriate setting of W g as shown in Inequality (1) prevents read signals from degrading. It allows one to write information signals on both guide grooves and lands of even erasable optical disk in good condition and provides high density recording. Secondly, the thickness of the record film is set to be relatively thin, between 5 nm and 50 nm because we found that if the thickness is greater than that the heat is not dissipated in the direction of the thickness but diffused to the next information track where record marks are partially erased. FIG. 11 illustrates a magnified top view of the optical disk to explain the next track erasure. Reference numerals 141, 142, 143, and 144 are a record mark, a guide groove, a land, and a beam spot, respectively. Beam spot 144 heats up record film 123 of land 143, which melts at the melting point and then turns into the amorphous phase during the cooling process thus forming a record mark. If the record film has a good thermal conductivity, the heat generated at land 143 by the beam spot to melt the record film there may diffuse to guide groove 142 and erase part of a record mark on groove 142. In the present invention the record film is specified to have a limited thickness so that the overwrite operation does not partially erase record marks on the next information track. Thirdly, as the reflection film is made of a metal film of a high thermal conductivity, the heat is more easily diffused in the direction of the film thickness. Consequently, erasure of record marks on the next tracks in overwrite operation is prevented by spreading the heat over a wider region of the record film. The preferred embodiments of the present invention will be described below. Embodiment 1 The optical disk of this embodiment has the same structure as shown in FIG. 3. Substrate 121 is a polycarbonate substrate having guide grooves formed in advance. Record film 123 is made of a GeSbTe alloy and 25 nm thick. Protection films 122 and 124 are made of a mixture of ZnS and SiO 2 including 20 mole % SiO 2 . Protection films 122 and 124 are 110 nm and 18 nm, respectively, in thickness. Reflection film 125 is made of Al and 150 nm thick. The depth of the guide groove is approximately 84 nm. FIG. 12 shows the read error rate as a function of W g /R for the optical disk used in this embodiment which has the same structure as shown in FIG. 3. It indicates that so as to meet the upper limit of 3% error rate of the CD Standard the W g /R must be between 0.34 and 1.0. When W g /R is less than 0.34, the beam spot overlaps with the adjacent tracks and partially erases record marks there giving rise to increase of error rate. When W g /R is greater than 1.0, the track width is wider than the beam spot and incomplete erase will take place in single-beam overwrite resulting in increase of error rate. We performed an experiment in which signals were written first on a guide groove and then on the adjacent lands one thousand times and the error rate of the signals of the guide groove was measured. The result was in agreement with the conclusion mentioned above. The measurement method of the error rate is described below. The EFM signals were written in the single-beam overwrite scheme on an optical disk used in this embodiment which was rotating at the rate of 1.25 m/sec. The signals were written first on a land and then on the adjacent guide grooves one thousand times and then the signals on the land were read. Compact Disk (CD) Standard uses the EFM signals which include nine kinds of pulses, 3T ~ 11T, (T represents a period) of different pulse widths (Refer to the International CD Standard IEC-908). The laser used for reading had a wavelength of 780 nm. The NA of the object lens was 0.55. We used the model DR-3553 of Kenwood Corporation, Ltd. and measured the number of block errors per second to plot the read error rate. Embodiment 2 FIG. 13 shows a structure of the optical disk of this embodiment. Substrate 121 is a polycarbonate substrate. Record film 123 is made of an alloy of GeSbTe. Protection films 122 and 124 are made of a mixture of ZnS and SiO 2 including 20 mole % SiO 2 and are 110 nm and 18 nm, respectively, in thickness. Wg is approximately 0.7 µm, the width of the taper portion approximately 0.1 µm, and the depth of the guide grooves approximately 84 nm. With this structure in this embodiment we measured the read error rate as a function of the thickness of the record film. FIG. 14 shows the result of the experiment. It indicates that so as to meet the upper limit of 3% error rate of the CD Standard the film thickness must be less than 50 nm. If it is thinner than 5 nm, the film poorly absorbs the laser beam while the thermal radiation is relatively large. Consequently, the write sensitivity (the laser power needed to form a record mark) decreases. Therefore, we must use a higher power laser source than a conventional diode laser, resulting in higher cost of lasers. Hence the film of thickness less than 5 nm is not a practical choice. For the measurement of the error rate we used the same method in this embodiment as that used in the embodiment 1. Embodiment 3 In this embodiment we measured the error rate as a function of the thickness of the record film of the optical disk including an Al reflection film having a high thermal conductivity as shown in FIG. 3. Substrate 121 is a polycarbonate substrate. Record film 123 is made of a GeSbTe alloy. Protection films 122 and 124 are made of a mixture of ZnS and SiO 2 including 20 mole % SiO 2 and are 110 nm and 18 nm, respectively, in thickness. Reflection film 125 is made of Al and 150 nm thick. W g is approximately 0.7 µm, the width of the taper portion approximately 0.1 µm, and the depth of the guide grooves approximately 84 nm. With the Al film playing a roll of a heat sink the heat can diffuse easily in the direction of the thickness, and it prevents thermal erasure of the next information tracks. FIG. 15 shows the result of the measurement of the error rate as a function of the thickness of the record film. It indicates that the error rate of this embodiment is superior to that shown in FIG. 14, where an Al film is not formed in the structure of the optical disk and that the thickness less than 70 nm meets the CD Standard. For the measurement of the error rate we used the same method in this embodiment as that used in the embodiment 1. Embodiment 4 In this embodiment we measured the error rate of the optical disk as shown in FIG. 3 as a function of the thickness of the Al film. Substrate 121 is a polycarbonate substrate. Record film 123 is made of a GeSbTe alloy and 25 nm in thickness. Protection films 122 and 124 are made of a mixture of ZnS and SiO 2 including 20 mole % SiO 2 and are 110 nm and 18 nm, respectively, in thickness. Reflection film 125 is made of Al. W g is approximately 0.7 µm, the width of the taper portion approximately 0.1 µm, and the depth of the guide grooves approximately 84 nm. FIG. 16 shows the result of the measurement of the read error rate as a function of the Al film thickness. The method of the measurement is the same as that described in the embodiment 1. FIG. 16 indicates that when the thickness exceeds 10 nm the heat sink effect as mentioned in the embodiment 3 sets in and the error rate starts decreasing. The reason for the larger error rate for the thinner thickness is that the small thermal capacity of the film cannot take away quickly enough the heat generated in the record film. The reflection films made of other metals (Au, Ti, Ni, Cr, and so on) demonstrated the same effect over the thickness of 10 nm.

Claims: An optical information write/read system in which information data is written on an optical disk by a write laser beam, and written information data is read from the optical disk by a read laser beam, said write laser beam having a beam diameter R at a focal point defined as a diameter where the intensity reduces to 1/e 2 of the intensity at the center of said write laser beam, and said read laser beam having a wavelength λ, said optical disk having a recording film, the reflectance of which locally varies when irradiated by a laser beam, and a plurality of tracks each comprised of a guide groove of a groove width and a region of a width equal to the groove width and positioned between the neighboring guide grooves, on which disk is recorded a signal by the utilization of a local change in optical constant or physical shape brought about by irradiation of a light beam, said groove width when converted into an optical length being equal to a value multiplied by a predetermined fraction of the wavelength of a recording and/or reproducing beam, wherein the guide grooves (101, 103, 105; 112, 114, 116; 132; 142) and regions (102, 104; 113, 115; 131, 133; 143) of said optical disk are used for information tracks, said system being characterised in that : the depth of said guide grooves in optical length is between approximately λ/8 and λ/4.7, and the width Wg of said guide grooves and the width of said regions between adjacent guide grooves are approximately equal to each other and satisfying the relationshsip 0.34≤ W g/ R ≤ 1.0. The optical information write/read system of claim 1, wherein the depth of said guid grooves of the recording medium in optical length is between approximately λ/7 and λ/5.2. The optical information write/read system of claim 1 or 2, wherein said record film (110; 123) of the optical disk is made of a phase-change type material that undergoes phase change between an amorphous phase and a crystalline phase. The optical information write/read system of claim 3, wherein a dielectric film (122), a record film (123), and another dielectric film (124) are formed in this order on a substrate (121) of the optical disk the thickness of said record film being between 5 nm and 50 nm. The optical information write/read system of claim 3, wherein a dielectric film (122), a record film (123), another dielectric film (124), and reflection film (125) are formed in this order on a substrate (121) of the optical disk, the thickness of said reflection film being not less than 10 nm. The optical information write/read system of claim 3, wherein a dielectric film (122), a record film (123), another dielectric film (124), and reflection film (125) are formed in this order on a substrate (121) of the optical disk, the thickness of said reflection film being between 5 nm and 70 nm. The optical information write/read system of claim 6, wherein the thickness of said reflection film (125) of the optical disk is not less than 10 nm.

Inventors: MIYAGAWA NAOYASU, NISHIUCHI KENICHI, OHNO EIJI, MIYAGAWA, NAOYASU, NISHIUCHI, KENICHI, OHNO, EIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: G11B7
IPC codes: G11B7, B41M5